The long term goal of this project is to study mechanisms of genetic recombination, in particular the mechanism of recombination of virus DNA with the host chromosome (site-specific-recombination). Recombination between bacteriophage Lambda and its host Escherichia coli is effected by a pair of reciprocal strand exchanges between specialized regions in each DNA called attachment sites. Int and IHF proteins specifically promote these exchanges. We have isolated and studied the phenotype of mutants of a central region of the Lambda attachment site. Certain mutations in this region disrupt homologous pairing between sites, while others disrupt protein interaction with the sites. We have isolated a plasmid that overproduces one of the peptides required for site specific recombination. Analysis of the structure of the attachment sites suggests that int protein recognizes a particular pattern of helical twist angle deviations in addition to a specific nucleotide sequence. We are also characterizing enzymes that promote homologous recombination. We find that endonuclease I of bacteriophage T7 cleaves Holliday structures (branched recombinational intermediates that arise by a reciprocal single-strand exchange). Cleavage occurs by nicking at the branch point.